SELECT 
  cscart_product_prices.product_id, 
  MIN(
    IF(
      cscart_product_prices.percentage_discount = 0, 
      cscart_product_prices.price, 
      cscart_product_prices.price - (
        cscart_product_prices.price * cscart_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  cscart_product_prices 
WHERE 
  cscart_product_prices.product_id IN (
    91827, 91796, 91825, 91824, 91823, 91808, 
    91807, 91806, 91805, 91804, 91803, 
    91639, 91600, 91537, 91535, 91425, 
    91025, 90894, 90888, 90881
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00084

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_prices range usergroup,product_id,lower_limit,usergroup_id usergroup 9 40 Using where

Result

product_id price
90881 1.69000000
90888 1.69000000
90894 1.69000000
91025 1.69000000
91425 1.69000000
91535 1.69000000
91537 1.69000000
91600 1.69000000
91639 1.69000000
91796 1.69000000
91803 1.69000000
91804 1.69000000
91805 1.69000000
91806 1.69000000
91807 1.69000000
91808 1.69000000
91823 1.69000000
91824 1.69000000
91825 1.69000000
91827 1.69000000